Analysis and low-complexity design for cellular and cell-free massive MIMO

Abstract

The advancements in technology have stimulated huge changes in modern communication networks worldwide. The rapid growth has produced huge demands for low cost, high data rates and reliability across a large number of active users. Massive multiple-input multiple-output (MIMO) has proven to be a successful technology to fulfil the demands required of the latest wireless communication systems, within the limited available bandwidth. Massive MIMO includes two types: cellular and cell-free massive MIMO. Cellular massive MIMO has become the core technology for fifth-generation (5G) wireless systems. On the other hand, cell-free massive MIMO is a new technology which is considered as one of the promising technologies for sixth-generation (6G) systems, due to its ability for increasing the network connectivity. Therefore, under developed areas of research for massive MIMO are of high interest for investigation. Much of the research performed on massive MIMO is completed assuming single-antenna user models. However, many modern devices can support multiple-antennas. Therefore, in this thesis, we focus on the performance analysis and low-complexity system designs of both cellular and cell-free massive MIMO using general models with multiple-antenna users. The thesis consists of two main parts: system designs for cellular massive MIMO and system designs for cell-free massive MIMO.


In the first part, we focus on cellular massive MIMO systems. A novel and low-complexity precoder based on maximum-ratio processing is proposed, with the objective of enhancing or enforcing the channel hardening phenomenon in massive MIMO systems. Additionally, the massive MIMO system is investigated for the keyhole channel and compared with Rayleigh fading channels. Massive MIMO under the effects of the keyhole channels is under researched, due to complications such as, the non Gaussian nature of the channel and the loss of channel hardening as the number of base station antennas grows large. The novel precoder is found to be effective at hardening the channels. More specifically, significant performance gains are produced by the proposed precoder in the keyhole channel with the condition of single-antenna users. The analysis of Rayleigh and keyhole channels for multiple-antenna users also concludes that: under Rayleigh fading channels, increasing the number of users or user antennas had no significant change in performance. Conversely, under keyhole channels, increasing the number of served users produces a much greater increase in performance compared to more user antennas.


In the second part, we focus on cell-free massive MIMO systems under practical hardware impairments. Cell-free massive MIMO systems are likely to experience in-phase quadrature-phase imbalance (IQI), due to the large number of inexpensive analog hardware with limited accuracy components. Motivated by this, the work of this thesis provides an analysis of cell-free massive MIMO systems with the presences of IQI, for both uplink and downlink. The results demonstrate that the performance of a cell-free system will saturate even if the number of access points goes to infinity. A compensation scheme to target the IQI is introduced and proved successful in removing this performance limit.

Date of Award	Jul 2023
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Queen's University Belfast
Supervisor	Michalis Matthaiou (Supervisor) & Hien-Quoc Ngo (Supervisor)